Tubes Asylum

Posted by Richard CWY (A) on May 28, 2002 at 16:29:36

In Reply to: Hot running Eico hf-86 amplifier posted by chopper87 on May 28, 2002 at 12:38:02:

It seems some thing went wrong if the power supply transformer (PST) gets too hot within minutes. Usually, after running a HF-86 for 1-2 hours, the temperature of the PST should be moderately hot, i.e. our bare hands can touch it without the felling of 'pain' (below 60 degree Celcius).

Overheating PST may result from:

a) The PST cannot provide the required current - stock PS tranie should be marked a Part No. likely be "30021", same as that in HF-81. Check if your PST is stock and working normally.

b) The output tubes (EL84s) draw abnormally high current - EL84 works happily within 35 - 45mA and so you can calculate the normal range of current pass through the cathod resistor (if my memory serves me well, it is 165 ohm/ 5W tolerance). Make sure that the value of this resistor is in order. Find out the current passing through the cathod resistor by messauring its voltage by Ohm's Law calculation. If too high, the negative bias may be wrong and the plate could turn red quickly.

Hope the above of some help to you.

Richard

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